1. Field of the Invention
The present invention relates to surveillance systems, and, more particularly, to surveillance systems for detecting an intruder in a monitored area of space.
2. Description of the Related Art
Surveillance systems for detecting intrusions of a moving object, such as a human, into a monitored area of space are known. The motion detectors often include infrared detectors that sense the presence of a source of infrared radiation, e.g., a warm body, anywhere along the line of sight of the infrared sensors.
A problem with infrared detectors is that they cannot easily distinguish between a human intruder and a house pet, such as a dog or a cat. It is particularly difficult for an infrared detector to distinguish between a pet at close range to the detector and a human located further away from the detector. An undesirable consequence of this problem is that an infrared detector may falsely set off an alarm in response to detecting a pet.
The detectors may also include microwave-based Doppler detectors that sense movement of objects by transmitting microwave energy and receiving the microwave energy after it has been reflected off of the objects. One problem with microwave-based Doppler detectors is that, similarly to infrared detectors, they sometimes cannot easily distinguish between a human intruder and a house pet. A small object close to the detector may produce the same signals as a larger object that is farther away from the detector. Thus, a dog that is fifteen feet from the detector may produce a signal similar to that of a human who is thirty feet from the detector. Like infrared detectors, microwave-based Doppler detectors may falsely set off an alarm in response to detecting a pet.
Another problem with microwave-based Doppler detectors is that they cannot easily distinguish between a human intruder and other inanimate objects that may have some movement, such as balloons, hanging signs, or curtains, all of which may be moved to some degree by air currents. Thus, a microwave-based Doppler detector may issue a false alarm as a result of detecting the movement of an inanimate object.
While various methods of reducing false alarms in intrusion detection systems have been developed there remains a need in the art to provide an intrusion detection system that can easily distinguish human intruders from both house pets, or other small animals, and moving inanimate objects, and that is thus less susceptible to issuing false alarms as a result of detecting house pets or moving inanimate objects.